[kwsantiago]

Running splicein (or a splice script) on a channel while the peer's splice was pending turned it into an RBF announced at a quarter of the intended feerate, and when the peer refused it the plugin's abort_channels made channeld abort the peer's splice, which we had already signed, and fail the channel. These changes keep a signed splice through a user abort and make RBFs of a pending splice follow BOLT 2's feerate rules.

Aborting:

• channeld never aborts an inflight it has sent tx_signatures for; an abort during a negotiation only drops the candidate that negotiation added.
• abort_channels refuses a channel whose splice is waiting only on the peer's tx_signatures (pointing at a reconnect, or close), aborts the other channels it was given, and returns them as refused. The splice plugin carries on to unreserve and report its own error when abort_channels fails.
• lightningd learns the peer's tx_signatures when they arrive on a reestablish resume, so it doesn't refuse an abort it no longer needs to.
• A confirmed_stfu arriving after its stfu_channels command has gone leaves STFU, or is ignored if a splice command now has the channel, instead of failing the channel or crashing lightningd.
• channeld forgets a splice_init it refuses, and lightningd frees an inflight's previous locked_scid when channeld updates it.
• When our broadcast of a splice completes, lightningd answers only the splice_signed that was pending when it was sent, not a splice command started later on the channel, whose caller got a splice_signed reply (the splice plugin crashed on it).

Feerates:

• The splice plugin sends feerates to lightningd as perkw, and weighs the channel input as channeld does when it checks our fee.
• Replacing a pending splice needs at least 25/24 of its feerate (or one more where that rounds back): splice_init uses it by default and refuses less, also under check, and refuses more than we'll pay unless force_feerate is set. The plugin takes the floor from listpeerchannels' next_feerate.
• channeld refuses a peer's splice RBF below that with tx_abort.

Each change has a test that fails without it, in tests/test_splicing.py and bitcoin/test/run-next_funding_feerate.c; dev options --dev-splice-warn-instead-of-sigs, --dev-splice-abort-after-sigs, --dev-splice-rbf-any-feerate and --dev-delay-confirmed-stfu reach the states they need.

Changelog-None